1. Technical Field
The present disclosure relates to the technical field of digital-analog converters and, in particular, to a current steering digital-analog converter.
2. Description of the Related Art
In modern applications with integrated circuits, for example in mobile terminals for personal communication, the need to minimize the space occupied by the integrated circuits, both in vertical and in horizontal directions, is increasingly felt. In the practice, the production of smaller and thinner packages or the vertical piling of multiple integrated circuits in a same package or in multiple superimposed packages is increasing. Solutions are adopted to obtain these results wherein the tendency is to minimize the thickness of the integrated circuit semiconductor wafers and use new types of materials for plastic packages and new types of glues for gluing the bottom portion of the dice. Nevertheless, it has been observed that these solutions subject silicon to stress phenomena whose effects cause greater consequences than the more or less negligible effects found when using packages of the previous generation.
For example, the effects of the above stress phenomena negatively affect the performance of current steering digital-analog converters.
As known, in this kind of converter a digital input code to be converted is used for driving an array of current generators obtained in the substrate and suitably sized. In particular, in this type of converter, a digital driving code is used for connecting/disconnecting a fixed number of current generators to/from a common summation node for providing an analog signal corresponding to the digital code to be converted in output in either a direct or indirect mode (in the latter case for example through an operational).
This type of digital-analog converters is particularly sensitive to gradual disturbances of various nature, for example due to:                a gradual variation of the oxide thickness affecting a considerable portion of the area occupied by the converter,        a voltage gradient on the masses that convey the currents to the converter, or        a gradual variation of the package pressure on the wafer by piezo-electric effect is such as to vary the mobility features of the components.        
An approach used in the known art for making the performance of a current steering converter less sensitive to such gradients consists in performing a scrambling of the positions of the various current generators of the converter in the portion of area intended for seating such generators, keeping each of the current generators concentrated in a portion of such area for reasons related to the ease of realization of the layout and to the advantage of area occupation.
The scrambling technique may further be applied together with the so-called thermometric coding of at least some of the bits of the digital code to be converted, generally the most significant bits, in order to improve the performance of the converters in terms of Differential Non-Linearity (DNL). This technique consists in the practice of replacing the digital input code with a more complex driving code, which also corresponds to the provision of a greater number of switches to be driven in the integrated circuit, so as to minimize the variations of the number of generators to be disconnected/connected from/to the common summation node for each unit increase/decrease of the digital code to be converted.
The Applicant has noted that the approach described above is such as to guarantee satisfactory performance of the converters if the disturbances have the features of a phenomenon affecting in a gradual and thus substantially deterministic manner the area occupied by the converter, and in particular occupied by the array of generators. However, such approach does not guarantee satisfactory performance if the integrated circuits are subject to strongly variable local and thus not gradual disturbances, which substantially lead to local alterations of the mobility and are of an accidental nature, that is, they are not predictable.
The Applicant has noted that such disturbances are a consequence of irregularities originating from the fillers used for the package or from glue blisters and are accentuated by the small thickness of the die with the contribution of the package pressure on the latter. Also such pressure is highly variable with temperature and increases at low temperatures, so this prevents effect compensation by a self-calibration of the converter when switched on because mobility variations are observed subsequent to temperature variations of the same converter during the operation thereof.
The need to provide a current steering digital-analog converter is therefore felt, capable of ensuring high performance even adopting the new packaging solutions described above, especially in the case wherein dice with particularly small thickness are used.